He was trying to control Bobby's posture by grabbing his leg, and my guess would be maybe trying to set up a guillotine down the road. There may not have been much thought put into it at all other than it was late in the round and he didn't want to get ground n pounded. If he left his arm where it was (on Bobby's left, instead of moving it to the inside on the right side of his head) that arm was pretty well isolated for other attacks, so it was kind of a pick your poison thing. I'm not flexible enough to try that Eddie Bravo type stuff, but Tony's probably more comfortable with that style of BJJ.
edit: and as far as I know, he didn't need to put his arm on that side of the head to try to control the posture.